This invention pertains to electrical machines for converting between electrical and mechanical energy and more importantly to a brushless, axial gap, air core electrical machine having low cost construction, and improved performance, efficiency and operating characteristics.
There is a need for low cost electrical machines for converting between electrical and mechanical energy that have both high efficiency and high performance capability. Unfortunately, current devices tend to suffer from one or more of a number of deficiencies, including low efficiency, low performance or speed capability, complex and expensive construction and poor heat transfer. One factor contributing to both the losses and complexity of many machines is the required use of laminations. Laminations reduce eddy current losses and the materials can be chosen for low hysteresis losses. However, the lamination losses still tend be around 3-4% or higher depending on the lamination thickness, material grade and rotor operating speed.
Another deficiency in many current designs is the result of slot winding. Slot winding allows for placement and structural support of armature windings, and a small air gap. However, these slots undesirably increase the inductance of the armature coils and also cause field variations during rotation, giving rise to additional losses.
Air core type electrical machines can eliminate some of these deficiencies. Unfortunately, current air core electrical machines still suffer from deficiencies that can include required use of laminations, low efficiency, poor heat transfer, and/or complicated, difficult or costly constructions. A better electrical machine having high efficiency, high performance and low costs without the deficiencies of current electrical machines is needed.
The invention provides a brushless electrical machine, for converting between electrical and mechanical energy, that has high efficiency, high performance and low cost construction. The electrical machine uses an air core armature for high efficiency, eliminates the need for laminations, has an axial gap air core armature for easy construction and assembly, and has a rotor construction suited for very high speed rotation. In some embodiments a controllable field coil is provided that can reduce the cost of drive electronics and provide added power control. In the configurations that use a field coil, the construction provides good support and heat transfer from the coil.
The electrical machine includes a rotor that rotates relative to a stationary stator. The rotor has two ferromagnetic rotor portions that are separated by a magnetic insulator. An axial airgap is formed between the ferromagnetic rotor portions with at least one of the rotor portions having axial protrusions facing the airgap. An axial air core armature is located in the axial armature airgap. The stator of the electrical machine includes a ferromagnetic yoke and a homopolar flux generator. The flux generator generates homopolar flux in the protrusions and across the armature airgap of the rotor by conducting flux through the yoke and to and from the two ferromagnetic rotor portions. The electrical machine eliminates the need for laminations because the stator field is homopolar and also because the axial armature airgap is formed between two sides of the rotating rotor instead of the rotor and stator. Further enabling the low losses, the ferromagnetic rotor portions and rotor-to-stator airgaps are designed to provide sufficient distance between the axial armature airgap and the yoke. The flux must have a large circumferential variation at the axial armature air gap for operation. The circumferential distribution of flux then becomes smoothed to be substantially uniform before entering the yoke, thereby reducing losses and eliminating the need for laminations.
The homopolar flux generator can be a field coil or a permanent magnet. Use of a field coil provides added control as a motor or generator and can reduce the cost of the brushless drive electronics. The field coil is mounted stationary with the yoke, which gives it both good mechanical support and heat transfer for continuous high power operation. If permanent magnets alone are used for the flux generation, they do not give the control benefits achieved with a field coil. However, they are located on the stationary yoke allowing high speed rotor rotation and can be located such that they bound the rotor-to-stator air gaps. In this location, they can further facilitate the flux in the stator yoke to be uniform for low loss. Axial magnetized magnets can be used for increased circumferential uniformity.
In one embodiment of the invention, the rotor is constructed from two solid ferromagnetic plates with the magnetic insulator located in between them. Without having a central hole, the rotor can be rotated to the highest rotational speeds due to lower stresses. In another embodiment, the ferromagnetic rotor portions are made of rings for reduced weight. The magnetic insulator can then be radially reinforced by the ferromagnetic rotor portions for high speed rotation. The magnetic insulator can be any material with low magnetic permeability and sufficient structural properties. Common materials include stainless steel and aluminum. Both have lower strength than high strength steel that can be used for high peripheral speed designs. In another embodiment, the magnetic insulator is constructed of ferromagnetic material and the insulative property is gained geometrically by having high reluctance between the two ferromagnetic rotor portions. In some configurations of the invention the rotor-to-stator air gaps are radial. This eliminates force between the rotor and stator when the rotor is centered in the stator. With axial rotor-to-stator gaps, the rotor can be axially centered to prevent unwanted force generation between the rotor and stator. In yet a further embodiment of the invention, the axial armature air gap is formed by a rotor constructed of an outer rim and an inner return path. A magnetic insulator ring separates the two ferromagnetic rotor portions. The outer rim has axial protrusions and forms the axial armature air gap with the inner return path.
The electrical machines in accordance with the invention can be used for numerous applications where high efficiency and low costs are desired, as well as for use in high speed devices. The invention also provides electrical machines that are axially compact, which may be desirable in some applications. Applications can include but are not limited to flywheel energy systems, generators, high speed machine tool drives, electric drive vehicles as well as common consumer products. The invention is particularly well suited for use in flywheel systems due to the high efficiency, high speed and power capability and low cost construction.